1. Field of the Invention
The present invention relates to a rotor blade provided with rotary spoilers and capable of controlling to reduce noise and vibration by disturbing airstream around the wing surface of the rotor blade to vary a lift of the rotor blade.
2. Description of the Related Art
Referring to FIG. 7, a helicopter is controlled to fly by a mechanical linkage 14 of a rotor 13, including an upper swash plate 10, a lower swash plate 11 and a pitch link 12. Efforts have been made to meet progressively increasing demand for reducing vibration and noise of the helicopter through the research and development of techniques for actively controlling a pitch of the rotor 13 in a higher frequency range.
U.S. Pat. No. 4,519,743 discloses a technique called an individual blade control method (IBC method) which is intended to achieve a high level control through not a swash plate but a hydraulic control of the respective rotor blade at the base end of each rotor blade. As mentioned in U.S. Pat. No. 5,224,826 studies have started from a rotor provided with a flap using a piezoelectric material, capable of efficiently working under the influence of a high centrifugal force, and having a better responsibility (responseability) beyond the limit of responsibility by the hydraulic control.
If the flight of the helicopter is controlled through the swash plates and the pitch link, the rotor generates vibratory force which vibrates the airframe of the helicopter, the lift of the rotor blades is caused to vary periodically according to rotation angle by the relation between the flying direction of the helicopter and the rotary angular position of the rotor blades, and the combined air vibrating effect of the rotor blades is exerted on the airframe to produce complex vibrational environment. FIG. 8 and 9 show a lift distribution on the rotor blade of the rotor 13 of the helicopter when the rotor blade is turning (rotating) in the flying direction of the helicopter and a lift distribution on the same rotor blade when the rotor blade is turning in a direction opposite to the flying direction thereof.
The control of vibrations through the direct, active control of the swash plate and the pitch link makes the complicated mechanical linkage more complicated, is practically disadvantageous in respect of weight and reliability, is capable of controlling only lower order of vibrations, and is incapable of satisfactorily effectively controlling the vibration generated by a rotor having four or more rotor blades.
The IBC method needs a rotating system including high-capacity hydraulic actuators for moving the rotor blades individually on a mast, which unavoidably increases the weight and makes the mast assembly complicated. The control operation of the IBC method which twists the whole rotor blades is not necessarily efficient.
The rotor provided with a flap is able to vary an effective portion with respect to the direction of a span or to control the effective portion partially. However, the rotor provided with a flap needs electrical control to achieve higher response. Since a large centrifugal force acts on a tip portion of the rotor blade, it is difficult to dispose a heavy device, such as a hydraulic actuator, on the tip portion of the rotor blade, and it is desirable to use a smart material, such as a piezoelectric material. However, piezoelectric materials can be strained only very slightly and hence an additional mechanism for amplifying the strain is necessary. If a flap is combined with the trailing edge of the rotor blade, small and lightweight flap hinges designed to exert only a low frictional resistance against the movement of the flap must be used.